C/EBPβ: A transcription factor associated with the irreversible progression of Alzheimer's disease

Abstract Background Alzheimer's disease (AD) is a neurodegenerative disorder distinguished by a swift cognitive deterioration accompanied by distinctive pathological hallmarks such as extracellular Aβ (β‐amyloid) peptides, neuronal neurofibrillary tangles (NFTs), sustained neuroinflammation, and synaptic degeneration. The elevated frequency of AD cases and its proclivity to manifest at a younger age present a pressing challenge in the quest for novel therapeutic interventions. Numerous investigations have substantiated the involvement of C/EBPβ in the progression of AD pathology, thus indicating its potential as a therapeutic target for AD treatment. Aims Several studies have demonstrated an elevation in the expression level of C/EBPβ among individuals afflicted with AD. Consequently, this review predominantly delves into the association between C/EBPβ expression and the pathological progression of Alzheimer's disease, elucidating its underlying molecular mechanism, and pointing out the possibility that C/EBPβ can be a new therapeutic target for AD. Methods A systematic literature search was performed across multiple databases, including PubMed, Google Scholar, and so on, utilizing predetermined keywords and MeSH terms, without temporal constraints. The inclusion criteria encompassed diverse study designs, such as experimental, case–control, and cohort studies, restricted to publications in the English language, while conference abstracts and unpublished sources were excluded. Results Overexpression of C/EBPβ exacerbates the pathological features of AD, primarily by promoting neuroinflammation and mediating the transcriptional regulation of key molecular pathways, including δ‐secretase, apolipoprotein E4 (APOE4), acidic leucine‐rich nuclear phosphoprotein‐32A (ANP32A), transient receptor potential channel 1 (TRPC1), and Forkhead BoxO (FOXO). Discussion The correlation between overexpression of C/EBPβ and the pathological development of AD, along with its molecular mechanisms, is evident. Investigating the pathways through which C/EBPβ regulates the development of AD reveals numerous multiple vicious cycle pathways exacerbating the pathological progression of the disease. Furthermore, the exacerbation of pathological progression due to C/EBPβ overexpression and its molecular mechanism is not limited to AD but also extends to other neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), and multiple sclerosis (MS). Conclusion The overexpression of C/EBPβ accelerates the irreversible progression of AD pathophysiology. Additionally, C/EBPβ plays a crucial role in mediating multiple pathways linked to AD pathology, some of which engender vicious cycles, leading to the establishment of feedback mechanisms. To sum up, targeting C/EBPβ could hold promise as a therapeutic strategy not only for AD but also for other degenerative diseases.


Results
6][27] The comprehensive investigation of C/EBPβ's cellular and subcellular localization in the healthy CNS is yet to be undertaken.Notably, C/EBPβ has been directly associated with the progression of AD in humans.Protein levels are heightened in the brains of AD patients in comparison to those of healthy controls.Furthermore, the transcript of Cebpb (which encodes C/EBPβ) exhibits greater abundance in the brains of aged AD mouse models than in those of aged control mice. 19The levels of C/EBPβ increase in neurons in an age-dependent fashion. 28ny pro-inflammatory genes contain the underlying C/EBPβ consensus sequence, which plays a crucial role in central nervous inflammation.In AD, glial cells manifest heightened C/EBPβ levels due to sustained upregulation of pro-inflammatory cytokines in the cortex, with a particularly notable increase observed in microglia. 29,30Microglia exhibit heightened expression and nuclear localization of C/EBPβ.Furthermore, immunohistochemical studies have validated robust colocalization between C/EBPβ-immunoreactive microglia and Aβ deposits and Aβ immunoreactive microvessels, along with pronounced immunoreactivity in pathologically vulnerable regions of the AD brain, contrasting with comparatively faint staining in corresponding areas of the non-demented (ND) brain or pathologically spared regions of the AD brain (e.g., cerebellum). 31merous studies have substantiated the pivotal role of C/EBPβ in the pathogenesis of AD, hastening its irreversible progression via diverse pathways (Table 1).

| The role of C/EBPβ in neuroinflammation and its functional interaction with inflammatory mediators
Neuroinflammation plays a pivotal role in the propagation of several neurodegenerative disorders and stands as a significant contributing factor in the pathogenesis and advancement of AD.
In the context of CNS inflammation, we primarily refer to the response of microglia and astrocytes to disruptions in homeostasis, which entail profound alterations in gene expression.These  32,33 Neuroglial cells assume a crucial role in various neurobiological processes, including neurogenesis, the preservation of neuronal well-being, and the formation of neuronal circuits.They accomplish these functions through a spectrum of activities, including anti-inflammatory responses, phagocytosis, steroid secretion, free radical scavenging, and cellular repair mechanisms. 34,35Neuroinflammation manifests in a state of heightened activation among glial cells, characterized by the production of pro-inflammatory cytokines.The excessive activation of inflammatory molecules intensifies neuroinflammation within the brain, precipitating the onset of AD and resulting in synaptic dysfunction, neuronal demise, and the impediment of neurogenesis, ultimately leading to irreversible cerebral damage. 36uroinflammation in the context of neurodegenerative diseases frequently manifests as a persistent and unresolved chronic process, contributing significantly to disease progression. 379 This suggests that C/EBPβ may inhibit some ERK1/2regulated genes while activating p38K-regulated genes, thereby influencing gene regulation. 48Overexpression of C/EBPβ isoforms LAP1 and LAP2 also have profound effects on the induction of C3-related gene promoter activity. 50,51In response to nerve injury, astrocytes secrete tissue inhibitors of metalloproteinase-1 (TIMP-1).Altered TIMP-1 levels have implications in various CNS diseases, including AD.Interestingly, overexpression of C/EBPβ in primary human astrocytes substantially enhances the activity of the −1718/+988 region of the TIMP-1 promoter, leading to an increased TIMP-1 expression. 44 which perpetuates the organism's state of disease. 52In summary, C/EBPβ, through its functional interactions with inflammatory factors, exerts regulatory control over a multitude of human astrocyte-related genes during neuroinflammation (Figure 1).

| C/EBPβ and microglia activation
5 Upregulation of C/EBPβ results in heightened iNOS expression via direct transcriptional mechanisms. 56,57e absence of C/EBPβ in microglia led to diminished expression of IL-1β and NO synthase-2, resulting in reduced NO production.
Additionally, it entirely abolished the neurotoxicity induced by cocultured microglia treated with LPS + IFNγ when in contact with neurons. 54Moreover, C/EBPβ appears to downregulate CD200R1 receptor expression in microglia during inflammation, rendering them less responsive to inhibitory CD200 ligands on neurons. 58The transcriptional regulation of macrophage colony-stimulating factor (M-CSF), a growth factor linked to inflammation and Aβ peptide formation in microglia, is orchestrated by C/EBPβ. 59,60C-reactive protein possesses the capability to activate the complement system and harbors a binding site for C/EBPβ within its promoter. 61eloperoxidase similarly contains a promoter site for C/EBPβ. 62e degradation of C/EBPβ in microglia resulted in the inhibition of neuroinflammation. 19C/EBPβ plays an essential role in upregulating inflammatory gene expression and inducing neurotoxicity in microglia during neurodegenerative diseases (Figure 1).It serves as a pivotal transcription factor for reprogramming microglial activation, making it a potential therapeutic target to mitigate neuronal injury caused by neuroinflammation.

| The C/EBPβδ -secretase axis is upregulated, resulting in increased levels of Aβ and NFT in AD
Asparagine endopeptidase (AEP), a cysteine protease, has been newly identified as δ-secretase.It cleaves APP and Tau in an agedependent manner, leading to the formation of amyloid plaques and NFTs in AD. 63 δ-secretase cleaves APP at residues N373 and N585 on the extracellular structural domain, enhancing Aβ production by BACE1. 64It also cleaves Tau at sites N255 and N368, leading to accelerated Tau hyperphosphorylation and the subsequent accumulation of NFT. 65Importantly, the tau (1-368) fragment is neurotoxic 66 and binds to, activating the transcription factor STAT1, thereby further up-regulating BACE1 transcription and Aβ production. 67Besides APP and tau, δ-secretase also cleaves polyanionic serine-arginine protein kinase 2 (SRPK2), a protein crucial in RNA splicing through the phosphorylation of SR splicing factors. 68During tau mRNA splicing, SRPK undergoes δsecretase cleavage, nuclear translocation, and an increase in kinase activity.This results in enhanced exon 10 attachment and an imbalance in 4R-tau and 3R-tau expression, ultimately promoting tau aggregation in tauopathy. 69Additionally, δ-secretase cleaves SET, which functions as a DNase inhibitor, PP2A inhibitor, and a regulator of tau phosphorylation. 70Fragments derived from δsecretase cleavage of SET lose their DNase inhibitor activity, induce genomic DNA incision, leading to neuronal cell death, 71 and inhibit PP2A activation, which triggers hyperphosphorylation and tau aggregation in AD 72 (Figure 2).Overexpression of δ-secretasederived SET fragments in the brain similarly reproduces the key features of AD in rats. 73In conclusion, it is widely validated that δ-secretase plays an important role in the pathogenesis of AD.

| C/EBPβ mediates the transcription of APOE4, the key gene in AD
ApoE, a glycoprotein primarily expressed in astrocytes, microglia, neurons, and the choroid plexus, functions as a lipid transporter, particularly for cholesterol and cholesteryl esters. 86In humans, the APOE gene exists in three polymorphic alleles (ε2, ε3, and ε4).APOE isoforms have been found to influence Aβ and Tau deposition in the brain, with APOE4 being the most potent genetic risk factor for AD. 87It substantially elevates the risk of both early-onset and late-onset AD in a dose-dependent manner. 88ApoE4-related pathogenesis can be categorized into Aβrelated and Aβ-independent mechanisms (Figure 3).ApoE plays a pivotal role in lipid transport and cholesterol homeostasis within the brain.Compared with other ApoE isoforms, ApoE4 isoforms show a low lipid state, are prone to form non-lipid-containing ApoE4 aggregates, and have poor lipid turnover between neurons and astrocytes, which leads to the accumulation of neuronal lipid droplets. 89This promotes mitochondrial damage and reactive oxygen species (ROS) production. 90In addition, ApoE4 also promotes the formation of endosomes and the degradation of synaptic receptors, including AMPA and NMDA, resulting in impaired synaptic function. 89,91Peripheral apoE4 may also have potentially negative effects on the pathological processes in brain development. 92ApoE can influence the neuroinflammatory process through various mechanisms; however, APOE4 astrocytes and microglia exhibit dysfunction in their response to inflammatory injury. 93Experimental results demonstrate that mice expressing the human ApoE4 isoform exhibit a higher presence of Aβ deposits and neuritic amyloid plaques compared to mice expressing other human ApoE isoforms. 94It has been reported that ApoE-isoforms have an Aβ-dependent effect on APP transcription and Aβ secretion.ApoE4 induces the strongest increase in APP transcription, 95 and ApoE4 also through mitogen-activated protein kinase signaling 96 and Aβ seeding and Aβ aggregation into oligomers and fibrils, increasing Aβ production and aggregation, 97 while apoE4 reduces Aβ clearance. 88Furthermore, ApoE4 exacerbates tau-mediated pathogenesis, neurotoxic neuroinflammation, and neurodegeneration. 98For instance, it increases tau neurotoxicity through its binding to the monoamine transporter 2 (VMAT2), inhibits the active transport of neurotransmitters into synaptic vesicles, and ultimately results in degeneration of the Locus Coeruleus. 99In a word, the ApoE4 is a significant factor in the development of Alzheimer's disease occurred.
ApoE production can be stimulated by a range of transcription factors, regulatory elements, hormones, cytokines, and lipids. 100A positive correlation exists between C/EBPβ expression levels and ApoE levels, indicating a close involvement of C/EBPβ in regulating APOE transcription and protein expression. 101The APOE promoter harbors multiple putative C/EBPβ binding motifs.Notably, the −305 to +93 fragment, which contains just one C/EBP-binding motif, exhibits transcriptional activity akin to that of the full-length promoter.
This fragment encompasses the primary C/EBPβ-binding site on the APOE promoter. 20In the mouse model of AD, C/EBPβ governs ApoE expression across multiple cell types, encompassing neurons, astrocytes, and microglia.Additionally, it modulates Aβ-induced ApoE transcription and protein expression, with a particular impact on ApoE4. 20C/EBPβ functions as an APOE-specific transcription factor by binding and interacting with the C/EBP-binding motif within the −305 to +93 fragment of the APOE promoter, facilitating the transcription of APOE genes (Figure 3).
Upon depletion of ER calcium stores, STIM1/2 detects reduced ER calcium levels, leading to oligomerization and subsequent translocation from ER-like sites to the plasma membrane, where they interact with calcium-conductive channels (ORAI and TRPC) to elicit calcium influx and replenish calcium stores. 110,111TRPC1, the first identified mammalian TRPC protein, exhibits widespread expression in both neuronal and nonneuronal tissues and is currently the principal candidate component of the SOCE system, contributing significantly to SOCE in various cell types. 112Cumulative evidence underscores the indispensable role of the TRPC1-SOCE pathway in the initiation and progression of neurodegenerative disorders.
Overexpression of full-length wild-type human tau (known as hTau) is an early tau pathological hallmark observed in sporadic AD.
The overexpression of intracellular tau can trigger TRPC1-dependent SOCE signaling by elevating TRPC1 mRNA and protein levels.This elevation leads to an increase in the steady-state intraneuronal calcium concentration ([Ca 2+ ] I), heightened ER stress, an imbalance in protein kinase and phosphatase activity, and enhanced tau protein phosphorylation at AT8 and p-S396, as previously documented. 22ER stress leads to the activation of AKT/GSK3β signaling, resulting in robust tau phosphorylation. 113Overexpression of hTau decreases the levels of PP2A-C (the catalytic subunit of PP2A), while PP2A-B (the regulatory subunit) remains unchanged, as demonstrated in previous research, 22 PP2A is known to be the primary tau phosphatase, regulating tau phosphorylation at various pathological sites, 114 This observation suggests that hTau overexpression diminishes PP2A activity, thereby promoting the neurodegenerative diseases, including AD, is causally related to cognitive decline. 119Histone acetylation and deacetylation processes are catalyzed by histone acetyltransferases (HATS) and histone deacetylases (HDACs), respectively.Additionally, cellular complex acetyltransferase inhibitor (INHAT) inhibits histone acetylation. 120,121Two key components of ANP32A (also known as I1 PP2A) and SET (also known as I2 PP2A), are selectively upregulated in brain regions affected by neurofibrillary pathology in AD. [122][123][124][125] ANP32A and SET together form an inhibitory complex, INHAT, which binds to histones and hinders their interaction with HAT. 126,127The prevention of ANP32A elevation in the hippocampal CA3 region rescues learning and memory decline in htau transgenic mice, and downregulating ANP32A effectively ameliorates synaptic impairment in these mice. 24Meanwhile, the phosphorylation and binding capacity of the microtubuleassociated protein tau are regulated by specific kinases and phosphatases. 128The activation of tau kinases, particularly GSK3β, plays a pivotal role in tau hyperphosphorylation in AD and related tauopathies. 1291][132] The ANP32A and SET act as endogenous PP2A inhibitors, leading to elevated tau hyperphosphorylation levels. 123,124Furthermore, the accumulation of AD-like mitogen-activated protein kinase (MAPT)/Tau disrupts autophagosome-lysosome fusion by perturbing the ANP32A-INHAT-IST1-ESCRT-III pathway (Figure 5).This finding highlights the role of ANP32A in mediating a detrimental cycle of MAPT/Tau accumulation and autophagy deficiency during the chronic progression of AD neurodegeneration. 120 further elucidate the underlying mechanisms contributing to the increased expression of ANP32A in AD, we discovered that ANP32A mRNA and protein levels were significantly elevated in response to hTau accumulation, Aβ1-42 exposure, or H 2 O 2 treatments. 24To identify potential regulatory elements, we conducted a screening of ANP32A promoter binding sites using the transcription factor binding database, 133 and we identified a conserved C/ EBPβ-binding site in the proximal promoter region of ANP32A. 24tably, AD-related stressors, such as tau overexpression, Aβ exposure, and oxidative stress, were found to enhance ANP32A gene transcription, with its expression under the regulation of C/ EBPβ 24 (Figure 5).Stimulation of C/EBPβ led to the overexpression of ANP32A. 134Additionally, AD-related stressors were observed to elevate the levels of total and phosphorylated C/EBPβ, which were also increased in the brains of AD patients and transgenic animal models of AD. 31,78 The upregulation of ANP32A in AD may be a

| C/EBPβ selectively triggers inhibitory GABAergic neuronal degeneration by repressing FOXO1
FOXO, a member of the forkhead family of transcription factors that acts downstream of insulin/insulin-like growth factor (IGF) signaling, plays conserved roles in longevity, cellular homeostasis, and cognitive performance. 1357][138] FOXO is phosphorylated by Akt and Mst1 at residues T24 and S212, respectively.FOXO1 phosphorylated by Akt is sequestered in the cytoplasm, where it inhibits apoptotic genes, such as BIM (a mediator of Bcl-2 cell death), promoting cell survival. 139In contrast, MST1-phosphorylated FOXO1 translocates to the nucleus, where it promotes apoptosis. 134 binding to its promoter. 16e up-regulation of neuronal C/EBPβ significantly affects insulin-triggered P-IR and p-FOXO1 T24/FOXO1 activity, especially in GABAergic neurons, which renders GABAergic neurons more vulnerable to damage and variability during aging. 23The loss of GABA and GABAergic interneurons in AD patients is a significant component of AD and may contribute to the network hyperactivity manifested as seizures. 142The onset of seizures frequently precedes cognitive decline and may serve as a precursor to AD. 143  LGMN mRNAs (Figure 6).Additionally, it has been found that C/EBPβ directly binds to the REST and FOXO1 promoters, functioning as a transcriptional repressor. 23 in PD patients. 149C/EBPβ preferentially mediates APOE4 expression, which would exacerbate α-syn pathology. 150In multiple sclerosis (MS), the expression level of C/EBPβ is increased, with LAP being the predominant isoform.Loss of C/EBPβ attenuates microglial cell viability and has a neuroprotective effect. 53Interaction between myelin basic protein-primed T cells and microglia activates NF-κB and C/EBPβ in microglia, leading to the specific expression of proinflammatory cytokines IL-1β, IL-1α, TNFα, and IL-1 in microglia. 151 amyotrophic lateral sclerosis (ALS), C/EBPβ has also been shown to act as a transcription factor regulating the expression of proinflammatory genes and is a candidate for regulating the expression of potentially neurotoxic genes in ALS microglia. 152

| C/EB Pβ REPRE S ENTS A PROMIS ING P OTENTIAL THER APEUTI C TARG E T FOR AD TRE ATMENT
C/EBPβ plays a pivotal role in the modulation of AD-related protein expression via multiple pathways, making it a crucial target for AD therapy.At present, drugs directly targeting C/EBPβ are relatively scarce in neurodegenerative disease research.However, microgliatargeting C/EBPβ inhibition has been shown to be a safe and effective method to reduce neurodegeneration driven by neuroinflammation. 53There are indications that drug-induced reduction of C/EBPβ levels may have neuroprotective effects.AD has a higher incidence in elderly women. 153

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Overexpression of C/EBPβ exacerbates the pathological features of AD, primarily by promoting neuroinflammation and mediating the transcriptional regulation of key molecular pathways, including δ-secretase, apolipoprotein E4 (APOE4), acidic leucine-rich nuclear phosphoprotein-32A (ANP32A), transient receptor potential channel 1 (TRPC1), and Forkhead BoxO (FOXO).Discussion: The correlation between overexpression of C/EBPβ and the pathological development of AD, along with its molecular mechanisms, is evident.Investigating the pathways through which C/EBPβ regulates the development of AD reveals numerous multiple vicious cycle pathways exacerbating the pathological progression of the disease.Furthermore, the exacerbation of pathological progression due to C/ et al.

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Presently, a multitude of studies have underscored the involvement of C/ EBPβ in the regulation of neuroinflammation, emphasizing its central role in the promotion of inflammatory gene expression.Notably, numerous pro-inflammatory gene promoters harbor putative C/EBPβ consensus sequences. 38Moreover, the expression and activation of C/EBPβ are subject to regulation by diverse extracellular signals, frequently involving intricate modulation by proinflammatory cues like IL-6, IL-43, and LPS.392.1.1 | C/EBPβ and astrocyte activation The activation of astrocytes is influenced by various stimuli, including lipopolysaccharide (LPS), interleukin-1β (IL-1β), and tumor necrosis factor-alpha (TNFα), which induce the expression of C/ EBPβ and C/EBPδ genes in primary astrocytes, thereby promoting astrocyte activation. 40,41Notably, LPS concentration dependently upregulates the expression of IL-1β and TNFα in astrocytes. 42The pro-inflammatory cytokine interleukin-1β (IL-1β) is one of the most potent and characteristic signals triggering astrocyte activation in most neurodegenerative diseases, mediating neuroinflammation through activation of glial cells and subsequent changes in gene expression. 43After IL-1β-mediated activation of glial cells, astrocyte C/EBPβ expression is induced by IL-1β and localized F I G U R E 1 C/EBPβ regulates signaling pathways associated with brain inflammation in AD.C/EBPβ and astrocyte activation: After the activation of astrocytes, the expression of C/EBPβ increases and migrates to the nucleus to regulate gene expression in the nucleus as a transcription factor.C/EBPβ inhibited some ERK1/2 regulatory genes, activated P38K regulatory genes, downregulated ERK1/2 expression, and increased p38K expression, respectively, BDKRB2 expression retention and cox-2 expression increase in astrocytes.C/EBPβ can also bind to C3, TIMP-1, and TGF-β1 promoters to promote their expression.C/EBPβ and microglia activation: After treatment with LPS, IL-1β, IL-6, or TNFα, C/EBPβ in microglia cells was significantly increased and its transfer to the nucleus was increased.C/EBPβ was bound to pro-inflammatory gene promoters in a stimulatory and gene-dependent manner to promote the expression of inflammatory genes in glia activation.iNOS, myeloperoxidase, C-reactive protein, M-CSF, and other promoters showed C/EBPβ-binding sites.Inflammatory factors activate glial cells, induce the increase of C/EBPβ expression, and promote the increase of the expression of more inflammatory factors.et al.

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regulatory mechanism of δsecretase has demonstrated that complete depletion of C/EBPβ results in the total elimination of δ-secretase expression.Within the −2029 to +3 base pair region of the LMGN (δ-secretase gene) promoter, the highest transcriptional activity is observed.C/EBPβ substantially enhances LMGN promoter activity and associates with the GAATTCAGAGGC sequence located 1480 bases upstream of the LMGN promoter, facilitating LMGN mRNA transcription74 (Figure2).C/ EBPβ serves as a crucial transcription factor that mediates the expression of δ-secretase, thereby promoting the accelerated development of Alzheimer's disease.The levels of δ-secretase protein in both the brain and spinal cord exhibit age-dependent increases.65In the CNS, C/EBPβ modulates the expression of δ-secretase in a time-dependent fashion.75,76Simultaneously, both Aβ and the δ-secretase-truncated APP C586-695 (C110) fragments stimulate the transcription of Alzheimer's disease-related genes by binding to and activating C/ EBPβ, which encompasses δ-secretase.77,78C/EBPβ plays a pivotal role in regulating the transcription and activation of LMGN mRNA during aging, thereby mediating the initiation and progression of AD pathology.79In parallel, δ-secretase engages in a feedback loop by cleaving APP and generating a protein-hydrolyzed APP C586-695 fragment, which selectively binds to and enhances the transcriptional activity of C/EBPβ.77C/EBPβ serves as the biological link in the detrimental cycle between δ-secretase and Aβ in the context of AD.Through the C/EBPβδ-secretase axis, C/EBPβ binds to enhancers in LMGN gene transcription and acts as a transcription factor, leading to an elevation in δ-secretase expression, thus expediting the progression of AD.Furthermore, the C/EBPβδ-secretase axis mediates the interaction between numerous exogenous and endogenous risk factors and the development of AD.Intestinal inflammation or F I G U R E 2 The molecular mechanisms underlying the C/EBPβδ-secretase signaling pathway expedite the progression of AD.The expression of δ-secretase is regulated by the transcription factor C/EBPβ, which is regulated by the BDNF/TrkB signaling pathway.δsecretase is phosphorylated by Akt, which inhibits its activity, and SRPK2, which enhances its activity.The δ-secretase cleaves APP and promotes the production of Aβ.The δ-secretase also cleaves Tau, producing fragments that are prone to aggregation.The tau fragment derived from δ-secretase-enhanced BACE1 activity, which further promoted the production of Aβ. δ-secretase-derived SET fragments inhibit tau dephosphorylation, while δ secretase-derived SRPK2 fragments affect tau selective splicing.Aβ and APP C586-695 (C110) fragments ultimately stimulate the transcription of genes associated with AD by binding to and activating C/EBPβ.All of these pathways promote tau aggregation and the occurrence of AD. et al.microbiota alterations activate the C/EBPβδ-secretase axis, initiating AD pathology in the gut, which subsequently ascends to the brain via the vagus nerve.28,80,81The concept of the gut-brain axis has garnered increasing attention, and the C/EBPβδ-secretase axis further strengthens the connection between intestinal inflammation or microbiota and neurodegenerative diseases.Atherosclerosis (ATH) and AD are both age-dependent inflammatory diseases associated with infiltrating macrophages, vascular pathology, and shared molecular factors.In recent experiments, it has been shown that C/EBPβ-AEP signaling links atherosclerosis (ATH) to AD by mediating vascular pathology.82Inflammatory responses triggered by a high-fat diet (HFD) activate neuronal C/EBPβ-AEP signaling, resulting in the development of AD pathology and cognitive impairments.83Traumatic brain injury (TBI) induces the activation of the transcription factor C/EBPβ, leading to an upregulation of δ-secretase expression.This, in turn, contributes to the pathogenesis of AD by fostering the production of Aβ, Tau hyperphosphorylation, and the induction of neuroinflammation and neurotoxicity. 84The expression level of C/EBPβ exhibited a negative correlation with BDNF/pTrkB signaling.Decreased BDNF/TrkB signaling results in elevated inflammatory cytokines and the activation of the JAK2/STAT3 pathway, ultimately leading to an upregulation of C/EBPβ and δ-secretase, consequently enhancing proteolytic APP and Tau fragmentation. 66,85Over recent years, numerous studies have substantiated C/EBPβ and δ-secretases as promising therapeutic targets for AD.
mediates the expression of APOE4, leading to the accumulation of lipid droplets and Aβ.The transcription factor C/ EBPβ binds and interacts with the C/EBP-binding motifs within the APOE promoter −305 to +93 fragment, acting as a specific transcription factor for APOE and promoting its gene transcription.ApoE4 can influence lipid transport through an Aβ-independent pathway, leading to mitochondrial damage and increased reactive oxygen species.Additionally, it can enhance Aβ production through an Aβ-dependent pathway, concurrently amplifying neuroinflammatory responses.Conversely, ApoE4 strongly activates C/EBPβ, subsequently enhancing δ-secretase cleavage of the increased APP.

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et al.accumulation of phosphorylated tau (p-Tau) in the brain.The overexpression of TRPC1 leads to an elevation in phosphorylated CaMKII level, a major kinase that targets the phosphorylation of tau at Ser262, which can be activated by increased intracellular[Ca 2+  ].115In summary, overexpression of intracellular hTau can induce ER stress through a TRPC1-dependent SOCE mechanism, resulting in the dysregulation of protein kinases and phosphatases, ultimately exacerbating tau pathology.Concurrently, TRPC1 overexpression leads to a significant reduction in dendritic spines within the hippocampal CA3 region, resulting in synaptic dysfunction and cognitive deficits.22SOCE-mediated NFAT1-NOX2-NLRP1 inflammasome corpuscle in LPS-induced neuronal damage and Aβ generation. 116C/EBPβ, a transcription factor associated with aging and listed in the GenAge database, is abundantly expressed in the brain and can be activated by risk factors associated with AD. 24 C/EBPβ was verified as a transcription factor responsible for mediating TRPC1 gene expression (Figure 4).Overexpression of hTau upregulated C/EBPβ expression, subsequently enhancing TRPC1 mRNA transcription and protein expression.Conversely, the downregulation of C/EBPβ reduced hTau-induced TRPC1 transcription and translation. 22The of hTau leads to an elevation in TRPC1 transcription by activating the transcription factor C/EBPβ, which in turn this upregulation of hTau occurs via the C/EBPβ-TRPC1-SOCE signaling pathway, thereby forming another vicious cycle of AD development.Activation of C/EBPβ induces transcription of the ANP32A gene, suppresses histone acetylation, and stimulates the formation of phosphorylated Tau Synaptic damage and memory impairment are hallmark pathologies and symptoms associated with Alzheimer's disease.Epigenetic modifications, particularly protein acetylation within the nervous system, play a crucial role in the regulation of gene expression related to long-term memory.117Acetylation of histones diminishes the electrostatic attraction between adjacent histones and DNA, thus fostering a more accessible chromatin configuration conducive to the transcription of genes associated with memory.118Histone acetylation, observed to be reduced in animal models of F I G U R E 4 C/EBPβ-TRPC1-SOCE signaling causes ER stress and dysregulation of protein enzymes and phosphatases.After G proteincoupled receptors and tyrosine receptors were activated, PLC was subsequently activated to generate IP3 from PIP2.As a calcium channel, IP3R binds to IP3 and is activated, resulting in calcium ion outflow.When Ca 2+ storage in the ER is depleted, STIM1/2 senses a reduction in Ca 2+ in the ER, which leads to oligomerization and subsequent transfer from ER-like sites to the plasma membrane, where it interacts with calcium conduction channels (ORAIs and TRPCs) to induce Ca 2+ influx and storage regeneration.In the context of hTau overexpression, activated C/EBPβ binds to the TRPC1 promoter sequence, promoting TRPC1 transcription, then increases neuronal steady-state [Ca 2+ ] I, enhances endoplasmic reticulum (ER) stress, imbalances protein kinases, and phosphatases, and exacerbates tauopathy.The C/EBPβ-Trpc1soce-signaling pathway in turn increases hTau, thus accelerating the progression of AD.

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consequence of increased transcription factors C/EBPβ, suggesting that the upregulation of C/EBPβ may represent a key role leading to ANP32A overexpression, followed by histone acetylation and impaired cognition.F I G U R E 5 The molecular mechanism of C/EBPβ-ANP32A-INHAT/PP2A affecting cognitive function.AD-related stressors increase total C/EBPβ levels and phosphorylated C/EBPβ levels.C/EBPβ induces ANP32 overexpression by binding to conserved recognition elements in the ANP32A proximal promoter region.ANP32A binds to SET to form an inhibitory complex (INHAT) that binds to histones and blocks their acetylation, ultimately resulting in reduced expression of life-related genes.ANP32A, as an endogenous PP2A inhibitor, can increase the tau hyperphosphorylation level.AD-like MAPT/Tau accumulation can inhibit autophagosome-lysosome fusion by regulating the ANP32A-INHAT-IST1-ESCRT-III pathway, and overexpressed Tau can further promote Cebpb gene expression.et al.
REST (RE1 silencing transcription factor, a transcription factor for FOXOs) and nerve activity are associated with IGF signaling, regulating the activity of the tripartite transcription factor.Nuclear REST and FOXO1 levels are strongly positively correlated with genes that repress excitation and synaptic functions, thereby delaying aging and extending lifespan. 140The activity of FOXO/DAF-16 (abnormal DAuer formation-16) is required to reduce the toxicity of Aβ aggregates under conditions of low insulin signaling, 141 indicating a potential protective role in AD.C/EBPβ has also been implicated in mediating IGF-1 and human insulin receptor (IR) expression by

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C/EBPβ overexpression inhibited the transcription of FOXO and REST genes by binding to their promoters, which not only makes GABAergic neurons more susceptible to degeneration during aging but also selectively induces the increased expression of LGMN in GABAergic neurons and increases the production of AEP in GABAergic neurons, which together lead to the aggravation of cognitive dysfunction in AD.MECHANIS MS OF AC TI ON OF C/EB Pβ IN OTHER NEURODEG ENER ATIVE DIS E A S E S C/EBPβ functions as a transcription factor, contributing to the progression of AD through diverse pathways.There is considerable F I G U R E 6 Molecular mechanisms of FOXO interact with C/EBPβ to accelerate AD progression.Overexpression of C/EBPβ inhibits REST and FOXO1 promoter activity and reduces REST and FOXO1 transcription.Decreased FOXO/DAF-16 (abnormal DAuer formation-16) activity increased insulin signaling, increased Aβ aggregation toxicity, and induced GABAergic neuronal degeneration and death.Inhibition of FOXO gene transcription can promote the transcription of LGMN, which leads to the increase of AEP and the accumulation of Aβ and Tau.experimental evidence that C/EBPβ can also contribute to the development of a variety of other degenerative diseases through several pathways.Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons and the presence of intraneuronal Lewy body (LB) inclusions, of which aggregated α-synuclein (α-syn) is a major component.C/EBPβ exhibits age-dependent regulation of MAOB (monoamine oxidase B) expression, a pivotal enzyme in dopamine metabolism, which can trigger oxidative stress in dopaminergic neu-rons and contribute to the aggregation of α-syn, a hallmark protein associated with neurodegenerative disorders.144Additionally, C/ EBPβ/AEP exhibits age-dependent activation in Parkinson's disease, wherein it assumes a role in mediating the presence of α-synuclein in both the gastrointestinal tract and the brain.145Deprivation of C/ EBPβ provided substantial in vitro and in vivo neuroprotection to dopaminergic cells, concurrently reducing inflammatory responses and glial activation levels.146The inactivation of the mitochondrial electron transport chain (ETC) activates the C/EBPβ/AEP pathway, thereby triggering the pathogenesis of PD.Deficiencies in all three complexes of the mitochondrial ETC robustly hinder oxidative phosphorylation, leading to a significant increase in ROS levels and activation of the C/EBPβ/AEP pathway, resulting in dopaminergic neuron cell death.C/EBPβ can also affect mitochondrial autophagy by regulating mitochondrial transcription factor A (TFAM).147These deficiencies also contribute to constipation and dyskinesia, key factors in PD pathogenesis.148BDNF and Netrin-1, which promote neuronal survival and regulate intestinal function, are strongly reduced in both the brain and gut of PD patients, and the deficiency of these nutritional factors in the gut causes dopaminergic neuron loss, constipation, and motor dysfunction.The transcription factor C/EBPβ, induced by inflammation and oxidative stress, acts as a potent repressor of BDNF and Netrin-1, suppressing the expression of nutritional factors, and thereby triggering non-motor and motor symptoms of PD.Its level exhibits a negative correlation with BDNF and Netrin-1

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CON CLUS IONC/EBPβ, activated by inflammatory cytokines, is a pivotal transcription factor regulating key enzymes and proteins across pathways that drive AD progression.Neuroinflammation is a prevalent hallmark of neurodegenerative diseases.C/EBPβ regulates proinflammatory gene expression during glial activation and plays a pivotal role in microglial activation, inducing neurotoxicity.54The δ-secretase, activated in the aging brain, is the key enzyme in the cleavage of APP and Tau, ultimately leading to cognitive impairment.63C/EBPβ et al.primarily upregulates LMGN gene transcription by binding to the CCCAGCTCTGGC promoter sequence, promoting δ-secretase expression.79APOE represents the most potent genetic risk factor for AD.C/EBPβ binds to the APOE promoter region (−305 to +93) and primarily mediates the expression of ApoE4 protein induced by Aβ.20 C/EBPβ-mediated vicious cycles occur in all three aforementioned pathways, reinforcing the irreversibility of AD progression.Furthermore, C/EBPβ functions as a transcription factor for the calcium channel protein TRPC1, enhancing C/EBPβ-TRPC1-SOCE signaling, leading to ER stress and dysregulation of proteases and phosphatases, resulting in increased phosphorylated hTau and exacerbating AD's pathological progression. 22ANP32A, a critical component of INHAT, mediates the vicious cycle between Tau/MAPT accumulation and autophagy deficiency, with its expression primarily regulated by C/EBPβ at the transcriptional level. 24Additionally, FOXO, a member of the FOXO transcription factor family, functions downstream of IGF-like signaling and can be selectively inhibited by C/EBPβ in degenerating GABA neurons. 23C/EBPβ promotes Aβ production and increases Tau phosphorylation through multiple pathways, resulting in elevated extracellular Aβ plaques and intraneuronal NFTs, thus mediating a vicious cycle of chronic neuroinflammation and neuronal loss.The overexpression of C/EBPβ expedites the irreversible advancement of AD pathophysiology.Furthermore, C/EBPβ plays a pivotal role in mediating various pathways associated with AD pathology, some of which develop vicious loops, resulting in the formation of feedback mechanisms.The significant involvement of C/ EBPβ in prevalent degenerative diseases such as PD, MS, and ALS has been established.Additionally, drugs that counteract C/EBPβ in cancer therapy have shown promising experimental outcomes.159For instance, Sesquiterpene lactone is frequently employed in cancer treatment research as a natural compound.Notably, Helenalin acetate can bind to the initial 21 amino acids of LAP*, disrupting its interaction with the coactivator p300 and consequently suppressing the transcription of the target gene.160Withaferin A and Celastrol have likewise been demonstrated to inhibit C/EBPβ activity through a mechanism similar to that of Helenalin acetate.161ST101, an innovative and selective peptide antagonist of C/EBPβ, binds to the leucine zipper domain of C/EBPβ, preventing its dimerization and augmenting the ubiquitin-proteasome-dependent degradation of C/EBPβ.162Research has demonstrated that LIP promotes cell survival during staurosporine-or taxol-induced Hep3B cell death.163Targeting LAP*/LAP could potentially result in LIP overexpression.Consequently, this review aims to enhance our comprehension of the interplay between C/EBPβ signaling and the expression of pertinent target genes.It also underscores the potential of C/EBPβ as a therapeutic target for addressing AD and other degenerative conditions.Currently, our understanding of the regulatory mechanisms governing gene expression in pathogenic glial cells remains limited, and the effectiveness and potential side effects of C/EBPβ antagonists in neurodegenerative diseases remain unclear.Researchers can continue to explore C/EBPβ as a focal point for extensive and in-depth investigations, with the aim of developing more efficacious treatment strategies and pharmaceutical interventions for AD, other neurodegenerative disorders, and even more challenging diseases.